Method of making SSZ-13 zeolites with high acidity and low silica to alumina ratio (SAR)

ABSTRACT

A method of forming an SSZ-13 zeolite in a hydrothermal synthesis yields an SSZ-13 zeolite that exhibits a silica to alumina (SiO2:Al2O3) molar ratio (SAR) that is less than 16:1; has a morphology that includes one or more of cubic, spheroidal, or rhombic particles with a crystal size that is in the range of about 0.1 micrometer (μm) to 10 μm. This SSZ-13 also exhibits a Brönsted acidity that is in the range of 2.0 mmol/g to 3.4 mmol/g as measured by ammonia temperature programmed desorption. A catalyst formed by substituting a metal into the framework of the zeolite provides for low temperature light-off of the NOx conversion reactions, while maintaining substantial performance at higher temperatures demonstrating hydrothermal stability.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing of International Application No. PCT/US2019/057804 filed on Oct. 24, 2019, designating the United States and published in English, which claims the benefit of the filing date under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 62/751,748 filed on Oct. 29, 2018, the entire contents of which are both incorporated herein by reference in their entirety.

This disclosure relates generally to a method of making an SSZ-13 zeolite that exhibits a low silica to alumina molar ratio (SAR) and high acidity, the SSZ-13 zeolite formed according to said method, and catalysts formed from the SSZ-13 zeolites. Such catalysts may be used for low temperature initiation or light-off of NO_(x) conversion reactions.

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. Microporous zeolites, which contain three-dimensional channels, play an important role in the selective catalytic reduction (SCR) of NO_(x) exhaust emissions that arise from diesel engines. More specifically, these zeolites are incorporated as part of an SCR catalyst that is capable of converting NO_(x) gases in the presence of a reducing agent (e.g., ammonia) to elemental nitrogen (N₂) and water (H₂O). In such SCR processes, the reducing agent is generally added to the exhaust gases prior to the gases contacting the SCR catalyst. The reducing agent absorbs onto the SCR catalyst and the NO_(x) reduction reaction takes place as the gases pass through or over the catalyst.

Zeolites are aluminosilicate materials that exhibit a crystalline framework of silica (SiO₂) and alumina (Al₂O₃) interconnected via the sharing of oxygen atoms. Thus, zeolites may be characterized by the ratio of SiO₂:Al₂O₃ (SAR) present in the crystalline framework. A higher SAR value generally correlates with the SCR catalyst exhibiting higher hydrothermal stability. Since an exhaust gas may contain water vapor and exhibit a temperature of at least 500° C. as it exits the engine, the hydrothermal stability of the SCR catalyst becomes an important consideration when designing the catalyst. However, the design of the catalyst must also take into consideration low temperature characteristics of the catalyst necessary to initiate or “light-off” the NO_(x) conversion.

SUMMARY

This disclosure relates generally to a method of making an SSZ-13 zeolite that has a low silica to alumina ratio (SAR) and high acidity, the SSZ-13 zeolites formed according to said method, and catalysts formed from the SSZ-13 zeolites.

According to one aspect of the present disclosure, the method of making an SSZ-13 zeolite comprises the steps of: a) providing a source of silica; b) providing a source of alumina; c) providing a structure directional agent (SDA); d) optionally, providing a source of a base; e) providing a source of metal cations; f) mixing the source of silica, alumina, SDA, and metal cations in water to form a gel composition; g) heating the gel composition to a crystallization temperature that is in the range of about 140° C. to about 180° C.; h) maintaining the gel composition at the crystallization temperature for a time period that ranges from 9 hours to 24 hours; i) allowing the SSZ-13 zeolite to crystallize and precipitate; the gel composition forming a crystalline precipitate of the SSZ-13 zeolite and a mother liquid; and j) separating the crystalline precipitate from the mother liquid.

The SSZ-13 zeolite so formed exhibits a silica to alumina (SiO₂:Al₂O₃) molar ratio that is less than 15:1. This method is a hydrothermal synthesis that yields an SSZ-13 zeolite with a morphology that includes one or more of cubic, spheroidal, or rhombic particles; and a crystal size that is in the range of about 0.1 micrometer (μm) to 10 μm. The SSZ-13 zeolite also exhibits a Brönsted acidity that is in the range of 2.0 mmol/g to 3.4 mmol/g as measured by ammonia temperature programmed desorption.

The SSZ-13 may be used as a support material for a catalyst for use in the low temperature NOx conversion. This catalyst may be prepared using a method that comprises: forming an SSZ-13 zeolite, such that it contains one or more metal cations as part of the SSZ-13 zeolite; providing a source of at least one transition metal; and subjecting the SSZ-13 zeolite and the source of the transition metal to an ion exchange, impregnation, or isomorphous substitution process. In this process, the at least one transition metal is substituted for the metal cations in the SSZ-13 zeolite, thereby forming the catalyst. The amount of the transition metal (TM) and aluminum present in the catalyst is further defined by a TM:Al molar ratio in the range of 0.5 to 1. The SSZ-13 zeolite is capable of low temperature initiation or “light-off” of the NO_(x) conversion.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:

FIG. 1 is graph of NOx conversion for a fresh catalyst and used catalyst plotted as a function of temperature demonstrating the definition of Light-off temperature;

FIG. 2 is a graph obtained from ammonia temperature programed desorption from which the Brönsted acidity exhibited by the SSZ-13 zeolites formed according to FIG. 4 is determined;

FIG. 3 is a flowchart of a method for preparing an SSZ-13 zeolite according to the teachings of the present disclosure;

FIG. 4 is a flowchart of a method for preparing a catalyst according to the teachings of the present disclosure using the SSZ-13 zeolite formed according to FIG. 1 ;

FIG. 5 is an x-ray diffraction (XRD) spectrum of the SSZ-13 zeolite formed according to the teachings of the present disclosure;

FIG. 6 is a scanning electron micrograph (SEM) of the SSZ-13 zeolite described in FIG. 4 ;

FIG. 7 is a table providing light-off temperature data measured for catalysts prepared with the SSZ-13 zeolite formed according to the teachings of the present disclosure; and

FIG. 8 is a chart of NOx conversion percentage plotted as a function of temperature for catalysts prepared according to the teachings of the present disclosure.

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is in no way intended to limit the present disclosure or its application or uses. For example, the zeolite support made and used according to the teachings contained herein is described throughout the present disclosure in conjunction with a selective catalytic reduction (SCR) catalyst in order to more fully illustrate the composition and the use thereof. The incorporation and use of such an SSZ-13 zeolite in other applications, such as adsorbents, ion exchange agents, or as a support material used for other industrial catalysts and/or environmental catalysts is contemplated to be within the scope of the present disclosure. It should be understood that throughout the description, corresponding reference numerals indicate like or corresponding parts and features.

The present disclosure provides a synthetic method for forming an aluminosilicate SSZ-13 zeolite capable of low temperature initiation or “light-off” of the NO_(x) conversion. The SSZ-13 zeolite formed according to the teachings of the present disclosure has a low SiO₂:Al₂O₃ molar ratio (SAR), for example, an SAR that is less than or equal to 16, while exhibiting a high level of acidity. This method limits the use of the structure directional agent (SDA) to an SDA to SiO₂ molar ratio that is between about 0.03:1 to about 0.05:1. The low amount of SDA utilized in this method along with the high yield of the SSZ-13 zeolite formed according to this method results in an inexpensive method that does not require additional steps, such as the reuse of the mother liquid in order to reduce cost.

The SSZ-13 zeolite formed according to the method described above and further defined herein is economically feasible for use in most applications. The prior use of conventional synthetic methods of forming an SSZ-13 zeolite and/or other CHA-type zeolites limited the use of such zeolites in cost sensitive applications, such as a support material for a catalyst in a selective reduction reaction (SCR) of NO_(x) contained in the exhaust gas of a vehicle.

According to one aspect of the present disclosure, the SSZ-13 zeolite formed according the method exhibits a SiO2/Al2O3 molar ratio of less than 15, an average particle size from 0.1 micrometer (μm) to about 10.0 μm, a morphology that includes one or more of cubic, spheroidal, or rhombic particles, and a high level of acidity. The SSZ-13 zeolite so formed has excellent durability and heat resistance, and metal-containing catalyst formed therefrom exhibit an enhanced NOx reduction rate at low temperatures as compared to metal-containing catalysts formed with conventional Chabazite-type zeolites. The SSZ-13 zeolite of the present disclosure is beneficial for use in application that require the low temperature light-off of NO_(x) conversion reactions.

For the purpose of this disclosure, light-off temperature refers to the temperature at which a desired catalytic reaction is initiated. For example, the light-off temperature may refer to, without limitation, the temperature at which NO_(x) conversion is initiated in a catalytic converter. Alternatively, the light-off temperature may be defined as the temperature wherein the NO_(x) conversion reaches at least 50% as shown in FIG. 1 for a fresh catalyst and used catalyst system.

In general, zeolites are crystalline or quasi-crystalline aluminosilicates comprised of repeating TO₄ tetrahedral units with T being most commonly silicon (Si) or aluminum (Al). These repeating units are linked together to form a crystalline framework or structure that includes cavities and/or channels of molecular dimensions within the crystalline structure. Thus, aluminosilicate zeolites comprise at least oxygen (O), aluminum (Al), and silicon (Si) as atoms incorporated in the framework structure thereof.

The SSZ-13 is an aluminosilicate zeolite that exhibits a framework topology that is the same as that of chabazite (framework notation “CHA”). Generally, an SSZ-13 zeolite exhibits a composition that includes a high ratio of silica to alumina (SAR), which leads to a low capacity to undergo cation exchange. However, in the present disclosure, the SSZ-13 zeolite formed exhibits a low silica to alumina ratio (SAR), thereby, allowing for a high capacity relative to cation exchange.

The notation, “CHA” represents a code specified by the International Zeolite Associate (IZA) that defines the framework structure of the zeolite. Thus, an SSZ-13 zeolite means an aluminosilicate in which the primary crystalline phase of the zeolite is “CHA”. In the SSZ-13 zeolite of the present disclosure, the presence of another crystalline phase or framework structure, such as “FAU”, in the zeolite is absent or nonexistent. In other words, the SSZ-13 zeolite of the present disclosure is substantially free of other crystalline phases and is not an intergrowth of two or more framework types.

In addition, the CHA crystal structure does not have an appreciable amount of phosphorous in its framework. In other words, the CHA zeolite of the present disclosure does not have comprise an amount of phosphorous that would affect the basic physical and/or chemical properties of the material, such as the zeolites capacity to selectively reduce NO_(x) over a broad temperature range.

The crystalline phase or framework structure of a zeolite may be characterized by X-ray diffraction (XRD) data. However, the XRD measurement may be influenced by a variety of factors, such as the growth direction of the zeolite; the ratio of constituent elements; the presence of an adsorbed substance, defect, or the like; and deviation in the intensity ratio or positioning of each peak in the XRD spectrum. Therefore, a deviation of 10% or less; alternatively, 5% or less; alternatively, 1% or less in the numerical value measured for each parameter of the CHA structure as described in the definition provided by the IZA is within expected tolerance.

According to one aspect of the present disclosure, the SSZ-13 zeolites may include natural zeolites, synthetic zeolites, or a mixture thereof. Alternatively, the SSZ-13 zeolites are synthetic zeolites formed according to the teachings of the present disclosure. Synthetic zeolites exhibit greater uniformity with respect to SAR, crystallite size, and crystallite morphology, as well has fewer and less concentrated impurities (e.g. alkaline earth metals).

The SSZ-13 zeolites comprise crystals with a mean crystal size of about 0.1 micrometers (μm) to about 10.0 micrometers (μm). Alternatively, the crystal size is between about 0.2 μm and about 8 μm; alternatively, at least 0.25 μm and less than 7.5 μm. The crystals in the SSZ-13 zeolites may be individual crystals, agglomeration of crystals, or a combination of both, provided the lower limit on the mean particle size associated with such agglomeration is the zeolites' mean individual crystal size.

Crystal size may also be referred to as a crystal diameter because it represents the length of one edge of a face of the crystal. Particle size of an agglomeration of crystals may be defined in a similar manner except that instead of the edge of a face of an individual crystal, the particle size represents the length of the longest side of the agglomeration of crystals. As used herein, the term “mean’ with respect to crystal or particle size is intended to represent the arithmetic mean of a statistically significant sample of the zeolites.

The SSZ-13 zeolites formed exhibit a mean particle size that is less than 50 micrometers (μm); alternatively, less than about 20 micrometers; alternatively, less than 10 μm. The mean particle size of the SSZ-13 zeolites may be measured using any known conventional method including, without limitation, laser diffraction, dynamic light scattering, and sieving.

According to another aspect of the present disclosure, the SSZ-13 zeolites have a mole ratio of silica-to-alumina (SAR) that is less than 16:1. Alternatively, the SAR of the SSZ-13 zeolites is between about 8:1 to about 15.5:1; alternatively, between about 10:1 to 15.5:1. The mole ratio of silica-to-alumina (SAR) of the zeolites may be determined using any conventional analysis means, such as for example, the x-ray fluorescence (XRF) or inductively coupled plasma (ICP) emission spectroscopy. The SAR ratio represents the ratio of silicon to aluminum atoms present in the crystal framework of the zeolite crystals.

The morphology exhibited by the “dried” SSZ-13 zeolites may resemble cubes, square flakes, spherical particles; rhombic particles, irregular particles, or a combination or mixture thereof. Alternatively, the morphology of the SSZ-13 zeolites include one or more of cubic, spheroidal, or rhombic particles.

The formed SSZ-13 zeolites may also exhibit a high level of acidity. The formed SSZ-13 zeolites exhibit a Brönsted acidity that is in the range of 2.0 mmol/g to 3.4 mmol/g as measured by ammonia temperature programmed desorption as shown in FIG. 2 . Alternatively, the Brönsted acidity exhibited by the SSZ-13 zeolites is in the range of about 2.5 mmol/g to about 3.0 mmol/g; alternatively, the Brönsted acidity exhibited by the SSZ-13 zeolites may be between about 2.7 mmol/g and about 2.8 mmol/g. In FIG. 2 , each of the two SSZ-13 zeolite samples (R-1, R-2) formed according to the teachings of the present disclosure and tested exhibit two NH₃ desorption peaks. The NH₃ desorption peak that occurs at a temperature that is less than 200° C. is associated with weak acid sites, while the other peak that occurs at a temperature that is higher than 500° C. is associated with strong acid sites.

Referring now to FIG. 3 , a hydrothermal synthetic method 1 is provided for producing an SSZ-13 zeolite that exhibits a silica to alumina (SiO₂:Al₂O₃) ratio of less than 15, a crystal size that is in the range of about 0.1 micrometers to 10 μm, and a morpohology that includes one or more of cubic, spheroidal, or rhombic particles.

This hydrothermal method 1 generally comprises the steps of:

-   -   a) providing 5 a source of silica;     -   b) providing 10 a source of alumina;     -   c) providing 15 a structure directional agent (SDA);     -   d) optionally, providing 20 a source of a base (OH⁻);     -   e) providing 25 a source of metal cations;     -   f) mixing 30 the source of silica, alumina, SDA, and metal         cations in water to form a gel composition;     -   g) heating 35 the gel composition to a crystallization         temperature that is in the range of about 140° C. to about 180°         C.;     -   h) maintaining 40 the gel composition at the crystallization         temperature for a time period that ranges from 9 hours to 24         hours;     -   i) allowing 45 the SSZ-13 zeolite to crystallize and precipitate         from the gel composition; forming a crystalline precipitate and         a mother liquid; and     -   j) separating 50 the crystalline precipitate from the mother         liquid.

The source of silica may comprise, consist essentially of, or consist of sodium silicate, silica sol, colloidal silica, fumed silica, tetraethyl orthosilicate, or a mixture thereof. The amount of silica present in in the gel composition is determined by the amount necessary for each of the other raw materials to be within the ranges specified herein with respect to the silica in order to provide an SSZ-13 zeolite that exhibits the desired SiO₂:Al₂O₃ ratio, crystal size, and morphology. The source of silica should be selected in a form that enables uniform mixing with the other raw materials used to form the gel composition.

The source of aluminum may comprise, consist essentially of, or consist of one or more of aluminum metal, aluminum hydroxide (e.g., gibbsite, boehmite, etc.), aluminum sulfate, aluminum nitrate, sodium aluminate, aluminum chloride, aluminosilicate gel, or a mixture thereof. The amount of alumina present in in the gel composition is determined by the amount necessary for each of the other raw materials to be within the ranges specified herein with respect to the silica in order to provide an SSZ-13 zeolite that exhibits the desired SiO₂:Al₂O₃ ratio, crystal size, and morphology. The source of alumina should be selected in a form that enables uniform mixing with the other raw materials used to form the gel composition.

The structure directional agents (SDA) that are used in the preparation of AEI-type zeolites are typically complex molecules capable of guiding or directing the molecular shape and pattern of the zeolite's framework. Generally, the zeolite crystals form around the SDA. After the crystals are formed, the SDA is removed from the interior structure of the crystals, leaving a molecularly porous cage-like structure.

The amount of the structure directing agent (SDA) utilized in forming the SSZ-13 zeolites is defined by a molar ratio of the SDA to silica (SDA:SiO₂) of about 0.03:1 to about 0.05:1; alternatively, about 0.04:1. The SDA may include, but not be limited to a hydroxide, a halide, a carbonate, a methyl carbonate, or a sulfate for which an N,N,N-trialkyladamantyl ammonium cation, an N,N,N-trimethylbenzyl ammonium cation, N-alkyl-3-quinuclidinol cation, or N,N,N-trialkylexoaminonorbomane cation is also present. Alternatively, the SDA is N, N-Dimethyl-3,5-dimethylpiperidinium hydroxide, N, N-Diethyl-2, 6-dimethylpiperidinium hydroxide, tetraethylphosphonium hydroxide, or a mixture thereof.

The amount of the structure directing agent (SDA) used to form the gel composition may also be characterized by the molar ratio of base (OH—) to silica (OH⁻:SiO₂) present in the gel composition, when the base is derived from the anion disassociated from the SDA when dissolved in the water. The molar ratio OH⁻:SiO₂ may range from about 0.6:1 to 1:1; alternatively, between about 0.7:1 to about 0.9:1; alternatively, about 0.8:1. Optionally, additional base (OH—) may be provided as a raw material used to form the gel composition provided the molar ratio of OH⁻:SiO₂ remains within the specified range.

The amount of water used in forming the gel composition may be defined by a molar ratio of water to silica (H₂O:SiO₂) that ranges from about 40:1 to about 50:1. Alternatively, the H₂O:SiO₂ molar ratio is between 42:1 to 48:1; Alternatively, the H₂O:SiO₂ molar ratio is about 45:1.

The source of metal (M) ions may comprise, consist essentially of, or consist of an alkali metal, an alkaline earth metal, or a combination thereof. The alkali metals or alkaline earth metals may be independently selected to be sodium (Na), potassium (K), calcium (Ca), barium (Ba), or strontium (Sr). The alkali or alkaline earth metal cations may be obtained from a metal halide, metal hydroxide, metal carbonate, metal hydrogen carbonate, metal nitrate, or metal sulfate. The inclusion of alkali metal or alkaline earth metal cations in the gel composition helps to facilitate crystallization by forcing the SDA to coordinate with aluminum in a preferred state. When a zeolite is used as an adsorbent or as a support for a catalyst, any alkali metal or alkaline earth atoms that are incorporated into the crystal structure of the zeolite during the formation of the zeolite may be removed from within the crystal structure by an ion exchange mechanism. An ion exchange mechanism is capable of replacing the alkali metal or alkaline earth cations with hydrogen, ammonium, or any other desired metal ion.

The reaction vessel used for hydrothermal synthesis is not particularly limited provided it is resistant to the application of heat and pressure. An example of such a vessel includes, without limitation, an autoclave. The mixture of raw materials charged into the reaction vessel may be sealed therein prior to heating and being exposed to an elevated pressure.

The gel composition formed in step f) of the method 1 in FIG. 3 may be subjected to hydrothermal conditions just after the preparation, or when desirable after undergoing a period of mixing, e.g., aging at a low temperature including, without limitation about room temperature or less than 100° C. over a period of about 1 to about 24 hours. During production on a large scale, a deterioration in the mixing the raw materials may be undesirable, in that a sufficient state of admixture is necessary to achieve high yield and proper crystallization of the SSZ-13 zeolites.

Still referring to FIG. 3 , during implementation of the method 1, the gel composition is subjected to heating 35 at predetermined crystallization temperature for a predetermined amount of time. This hydrothermal synthesis utilizes a crystallization temperature that is in the range from about 140° C. up to 180° C.; alternatively, between about 150° C. and about 170° C.; alternatively, from about 160° C. to about 165° C. The period of time over which the temperature is maintained 35 in order to result in the crystallization and precipitation of the SSZ-13 zeolite is between 9 hours and 24 hours; alternatively between about 12 hours and about 22 hours; alternatively between about 16 hours and about 20 hours.

Upon completion of the hydrothermal reaction, the SSZ-13 zeolite in the form of a crystalline precipitate is separated from remaining liquid (e.g., the mother liquid). The mother liquid may be discarded, or when desirable, reused as a replacement for at least a portion of the water that is used in the making of another batch of the SSZ-13 zeolite. This separation may use any known conventional method, including but not limited to, filtration, decantation, or direct drying (e.g., evaporation).

After separation from the mother liquid, the SSZ-13 zeolite, which may include some SDA and/or metal cations, may be collected, optionally washed with water, and then dried. The dried support material may be used in the dried state for some applications or subjected to calcination prior to use for other applications. Calcination of the SSZ-13 zeolites at a high temperature (e.g., >200 C; >300 C, etc.) removes any residual SDA present in the porous structure.

The yield of SSZ-13 zeolites formed according to this method is greater than 20% relative to the total oxide present in the gel composition. Alternatively, the yield is greater than 30%; alternatively, greater than 40%. Thus, the method of the present disclosure does not need to reuse the mother liquid as part of the water used to form the gel composition in order to obtain a high yield. However, since the mother liquid contains unreacted SDA, when desirable, the mother liquid may be used to replace at least a portion of the water in which the raw materials are mixed to form the gel composition.

The “dried” SSZ-13 zeolites formed herein may also exhibit a BET specific surface area that is greater than 500 m²/g; alternatively, at least 600 m²/g; alternatively, equal to or greater than 700 m²/g. The specific surface area of the SSZ-13 zeolites may be measured using a conventional Brunauer-Emmett-Teller (BET) method.

The use of the SSZ-13 zeolite formed according to the method of the present disclosure may include, without limitation, as a support material for a catalyst, an absorbent, or a separation material. The “dried” SSZ-13 zeolites may be used prior to or after calcination.

A catalyst may comprise the SSZ-13 zeolite with one or more catalytic metal ions exchanged for an atom in the framework or otherwise impregnated into the pores and/or cavities of the zeolite. As used herein, the terms “metal exchanged” and “metal substituted” are used in a similar fashion to mean that one or more of the aluminum or silicon framework atoms have been replaced by a substituting metal.

Several examples of catalytic metal ions that may be incorporated into the SSZ-13 zeolite include, without limitation, ions of transition metals, platinum group metals (PGM), precious metals, such as gold or silver; rare earth metals, or mixtures thereof. As used herein transition metals are elements of Groups 3-12, where the electron configuration starts filling electrons in the (3-6)d shells, while rare earth metals are elements that start filling electrons in the 4f or 5f shells. These d-block elements are generally called “transition metals”, while the f-block elements are generally called, lanthanides, actinides, and/or “inner transition metals”.

The transition metals may comprise, consist essentially of, or consist of copper, nickel, zinc, iron, tungsten, molybdenum, cobalt, titanium, zirconium, chromium, manganese, or tin. Alternatively, the transition comprises, consists of, or consists essentially of iron (Fe), copper (Cu), or manganese (Mn). Platinum group metals may include, without limitation, ruthenium, rhodium, palladium, indium, and platinum. Rare earth metals include lanthanum, cerium, praseodymium, neodymium, europium, terbium, erbium, ytterbium, and yttrium.

According to another aspect of the present disclosure a method of preparing a catalyst using the SSZ-13 zeolite as a support is provided. Referring now to FIG. 4 , this method 100, generally comprises the steps of: forming 105 an SSZ-13 zeolite according to the method previously described above and further defined herein that contains one or more metal cations as part of the SSZ-13 zeolite; providing 110 a source of at least one transition metal; and subjecting 115 the SSZ-13 zeolite and the source of the transition metal to an ion exchange, impregnation, or isomorphous substitution process, such that at least one transition metal is substituted for the metal cations in the SSZ-13 zeolite, thereby forming the catalyst.

The amount of the catalytic metal ions (M) and aluminum present in the catalyst is further defined by a molar ratio of the substituted metal to aluminum (M:Al) that is in the range of 0.5:1 to 1:1. Alternatively, the molar ratio of M:Al is in the range of about 0.6:1 to 0.9:1; alternatively, the molar ratio of M:Al is between about 0.7:1 and 0.8:1. The amount of catalytic metal ions that are incorporated into zeolite is capable of achieving a NO_(x) conversion of at least about 50%; alternatively, at least about 55%; and alternatively, at least about 60%, at a light-off temperature of at least about 210° C.; alternatively, at least about 220° C.; alternatively, at least about 230° C.; alternatively, at least about 250° C.; alternatively, at least about 270° C.

A higher metal loading incorporated into or on the SSZ-13 zeolite support generally results in higher NOx conversion. However, the level of metal incorporated therein is often dependent on the quantity of exchange sites that are present in the zeolite, which in turn is dependent upon the zeolite's SAR value. In general, zeolites with low SAR allow for the highest metal loadings, while zeolites with a high SAR exhibit higher hydrothermal stability. Thus, a conflict exists between the need for high catalytic activity and high hydrothermal stability. The catalyst formed according to the teachings of the present disclosure generally exhibits a high catalytic activity at a low operating temperature, e.g., below 400° C., while maintaining substantial hydrothermal stability.

The catalyst may be used in the form of a washcoat coated onto a substrate, such as a metal or ceramic flow-through monolith substrate or a filtering substrate. The monolith substrate may have a honeycomb geometry, which comprises a plurality of adjacent, parallel channels that have a square cross-sectional area. The honeycomb shape of a monolith substrate provides a large catalytic surface having a minimal overall size and pressure drop. A specific example of such a substrate includes, without limitation, a wall-flow filter. Accordingly, another aspect of the disclosure is a washcoat that comprises a catalyst component as described herein. In addition, the catalyst washcoat may further comprise a binder material, such as alumina, silica, (non-Zeolite) silica-alumina, naturally occurring clays, TiO₂, ZrO₂, and SnO₂.

According to another aspect of the present disclosure a process for reducing NOx emissions in a gas stream comprises contacting the emissions with a substrate upon which the catalyst has been deposited. This catalyst being a metal-containing SSZ-13 zeolite formed according to the teachings of the present disclosure. When desirable, the gas stream may be contacted with the exhaust article in the presence of ammonia, thereby providing a selective catalytic reduction (SCR) system.

The following specific examples are given to illustrate the disclosure and should not be construed to limit the scope of the disclosure. Those skilled-in-the-art, in light of the present disclosure, will appreciate that many changes can be made in the specific embodiments which are disclosed herein and still obtain alike or similar result without departing from or exceeding the spirit or scope of the disclosure.

In the following examples, a HORIBA LA-920 laser particle sizer is used for the measurement of particle size distribution, a Rigaku MiniFlex II DESKTOP X-ray diffractometer is used for the measurement of phase and crystallinity, a Micromeritics TriStar II 3020 is used for the measurement of BET surface areas, a Spectro Analytical Instruments Model FCPSA83D ICP is used for analysis of chemical compositions, and zeolite morphology is measured using scanning electron microscopy (SEM).

Example 1—Preparation & Characterization of a Batch of SSZ-13 Zeolites

A predetermined amount of aluminum sulfate (i.e., source of aluminum oxide) was added to water to form a clear solution. Then predetermined amounts of potassium hydroxide (i.e., source of metal cations) and N,N,N-trialkyl adamantyl ammonium hydroxide (i.e., structure directional agent, RNOH) were added into the solution. Finally, colloidal silica was added to the solution. The solution was mixed together to form a transparent sol or gel composition. The amount of each of the materials or components that form the gel described above were selected such that the final composition of the gel was 46K₂O:6.5Al₂O₃:100SiO₂:5000H₂O:3RNOH. The gel composition was transferred to an autoclave and heated to 165° C. for 24 hours. The SSZ-13 zeolite, which formed and precipitated, was filtered away from the mother liquid, washed with water, and dried at 120° C. The collected SSZ-13 zeolite was stored for further analysis and labeled R-3. The collected SSZ-13 zeolite (R-3) was found to exhibit an SAR value of 15.4.

Referring now to FIG. 5 the measured x-ray diffraction (XRD) pattern 200 for the zeolite (R-3) collected and dried in this example shows 2 theta peaks 205 that correspond to an SSZ-13 structure or framework. The measured XRD pattern 200 further demonstrates that this SSZ-13 zeolite is substantially free of any other type of crystalline zeolite phase or structure.

Referring now to FIG. 6 , the morphology of the SSZ-13 zeolite (R-3) collected and dried in this Example is shown in the scanning electron micrograph to include predominantly cubic structures 210 each having a length (L) on each side that is approximately 1200 nanometers (nm).

The SSZ-13 zeolite (R-3) collected and dried in this Example was also found to exhibit a BET specific surface area (SSA) on the order of about 516 m²/g and a power volume (PV) of about 0.3 cm³/g. Brunauer, Emmett, and Teller (BET) surface area, total porosity, and pore size distribution of fresh and/or aged samples of the zeolite may be measured using a Micromeritics TriStar II 3020 analyzer. Such a BET surface area may be in the range of about 100 to 800 m²/g, alternatively in the range of about 200 to 600 m²/g. The SSZ-13 zeolite may have a pore volume within the range of about 0.1 to 2 cm³/g; alternatively within the range between about 0.2 to 1.5 cm³/g.

Example 2—Measurement of Light-Off Temperature NOx Conversion

The light-off temperature is measured by evaluating the temperature at which 50% of the NOx has been converted to N₂ or other nitrogen species. A catalyst (R-4) was formed by substituting a portion of the metal cations in the SSZ-13 zeolite (R-3) with copper metal. The light-off temperature was then measured for a fresh sample of the catalyst (R-4) and then reexamined for samples of the catalyst (R-4) after being hydrothermally aged at a temperature of 700° C. or 825° C. for a period of 16 hours. Referring to the table shown in FIG. 7 , the light-off temperature exhibited by the fresh catalyst (R-4) was about 212° C., while the light-off temperature exhibited by hydrothermally aged catalysts (R-4) was slightly higher at 230° C. to 270° C.

Example 3—Measurement of NOx Conversion

The NO_(x) conversion associated with the copper containing catalyst (R-4) was measured utilizing a Micromeritics 2920 acting as a micro flow reactor and a MKS Residual Gas analyzer, which performs the analysis of gas concentrations after passing through the sample. The samples are typically tested with the follow gas concentrations: NO=175 ppm; NO₂=175 ppm; NH₃=350 ppm; and O₂=175 ppm. The samples are tested under 50,000⁻¹/hr space velocity. The NO_(x) conversion is measured at each individual temperature under steady state reactions, the sample is first heated to the target temperature and the NO_(x) conversion is then measured once steady state is observed. Typically, the NO_(x) conversion numbers are calculated from a steady state condition after 45 minutes of consistent pressure through the MKS Residual Gas analyzer.

Referring now to FIG. 8 , the fresh copper catalyst (R-4) formed by substituting a portion of the metal cations in the SSZ-13 zeolite (R-3) with copper metal results in about 100% conversion over a wide temperature window from 310 to 650° C. After hydrothermal aging at 700° C. for 16 hours, the catalyst (R-4) still has greater than 95% NOx conversion from 360 to 610° C. After hydrothermal aging at 825° C. for 16 hours the catalyst (R-4) is still alive for NOx conversion with greater than 80% NOx conversion in temperature from about 350° C. to about 550° C.

For the purpose of this disclosure, the terms “about” and “substantially” are used herein with respect to measurable values and ranges due to expected variations known to those skilled in the art (e.g., limitations and variability in measurements).

For the purpose of this disclosure any range in parameters that is stated herein as being “between [a 1^(st) number] and [a 2^(nd) number]” or “between [a 1^(st) number] to [a 2^(nd) number]” is intended to be inclusive of the recited numbers. In other words the ranges are meant to be interpreted similarly as to a range that is specified as being “from [a 1^(st) number] to [a 2^(nd) number]”.

For the purpose of this disclosure, the term “weight” refers to a mass value, such as having the units of grams, kilograms, and the like. Further, the recitations of numerical ranges by endpoints include the endpoints and all numbers within that numerical range. For example, a concentration ranging from 40% by weight to 60% by weight includes concentrations of 40% by weight, 60% by weight, and all concentrations there between (e.g., 40.1%, 41%, 45%, 50%, 52.5%, 55%, 59%, etc.).

For the purpose of this disclosure, the terms “at least one” and “one or more of’ an element are used interchangeably and may have the same meaning. These terms, which refer to the inclusion of a single element or a plurality of the elements, may also be represented by the suffix “(s)” at the end of the element. For example, “at least one metal”, “one or more metals”, and “metal(s)” may be used interchangeably and are intended to have the same meaning.

Within this specification, embodiments have been described in a way which enables a clear and concise specification to be written, but it is intended and will be appreciated that embodiments may be variously combined or separated without parting from the invention. For example, it will be appreciated that all preferred features described herein are applicable to all aspects of the invention described herein.

Those skilled-in-the-art, in light of the present disclosure, will appreciate that many changes can be made in the specific embodiments which are disclosed herein and still obtain alike or similar result without departing from or exceeding the spirit or scope of the disclosure. One skilled in the art will further understand that any properties reported herein represent properties that are routinely measured and can be obtained by multiple different methods. The methods described herein represent one such method and other methods may be utilized without exceeding the scope of the present disclosure.

The foregoing description of various forms of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Numerous modifications or variations are possible in light of the above teachings. The forms discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various forms and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled. 

What is claimed is:
 1. A method of forming a catalyst for use in low temperature NOx conversion, wherein the method comprises the steps of: forming an SSZ-13 zeolite containing one or more metal cations as part of the SSZ-13 zeolite according to a method comprising the steps of: a) providing a source of silica; b) providing a source of alumina; c) providing a structure directional agent (SDA); wherein the amount of the structure directing agent (SDA) and the silica (SiO₂) source provided is defined by a SDA:SiO₂ molar ratio that ranges from 0.03 to 0.05; d) providing a source of metal cations; e) mixing the source of silica and alumina with the SDA and the source of metal cations in water to form a gel composition; f) heating the gel composition to a crystallization temperature that is in the range of about 140° C. to about 180° C.; g) maintaining the gel composition at the crystallization temperature for a time period that ranges from 9 hours to 24 hours; h) allowing the gel to crystallize and precipitate; the gel composition forming a crystalline precipitate of the SSZ-13 zeolite and a mother liquid; and i) separating the crystalline precipitate from the mother liquid; wherein the formed SSZ-13 zeolite has a silica to alumina (SiO₂:Al₂O₃) molar ratio (SAR) that is less than 16:1; a morphology that includes one or more of cubic, spheroidal, or rhombic particles; and a crystal size that is in the range of about 0.1 micrometer (μm) to 10 μm; providing a source of at least one transition metal; and subjecting the SSZ-13 zeolite and the source of the transition metal to an ion exchange, impregnation, or isomorphous substitution process; wherein the at least one transition metal is substituted for the metal cations in the SSZ-13 zeolite, thereby forming the catalyst; wherein the amount of the transition metal (TM) and aluminum present in the catalyst is further defined by a TM:Al molar ratio in the range of 0.5 to 1.0.
 2. The method of claim 1, wherein the SSZ-13 zeolite exhibits a silica to alumina (SiO₂:Al₂O₃) molar ratio (SAR) that is in the range 8:1 to 15.5:1.
 3. The method of claim 1, wherein at least one of the source of silica is selected from the group consisting of sodium silicate, silica sol, colloidal silica, fumed silica, tetraethyl orthosilicate, and mixtures thereof.
 4. The method of claim 1, wherein the source of alumina is selected from the group consisting of aluminum metal, aluminum hydroxide, aluminum sulfate, aluminum nitrate, sodium aluminate, aluminum chloride, aluminosilicate gel, and mixtures thereof.
 5. The method of claim 1, wherein the source of alumina is selected as gibbsite or boehmite.
 6. The method of claim 1, wherein the amount of the water (H₂O) and the silica (SiO₂) source provided is defined by a H₂O:SiO₂ molar ratio that ranges from 40 to
 50. 7. The method of claim 1, wherein the method further comprises adding a base, such that the molar ratio of OH—to:SiO₂ is maintained in the range of 0.6 to 1.0.
 8. The method of claim 1, wherein the structure directional agent (SDA) is N,N,N-trialkyl adamantyl ammonium hydroxide.
 9. The method of claim 1, wherein the source of metal cations is selected from a group consisting of an alkali metal, an alkaline earth metal, and mixtures thereof.
 10. The method of claim 9, wherein the alkali metal and alkaline earth metal are independently selected from sodium (Na), potassium (K), calcium (Ca), barium (Ba), and strontium (Sr).
 11. The method of claim 1, wherein the formed SSZ-13 zeolite exhibits a Brönsted acidity that is in the range of 2.0 mmol/g to 3.4 mmol/g as measured by ammonia temperature programmed desorption.
 12. The method of claim 1, wherein the gel composition is heated to a crystallization temperature that is in the range of about 140° C. to about 180° C.
 13. The method of claim 1, wherein the gel composition is held at the crystallization temperature for a time period that ranges from about 9 hours to about 24 hours.
 14. The method of claim 1, wherein the formed SSZ-13 zeolite exhibits an average crystal size between about 0.25 micrometers to 7.5 micrometers.
 15. The method according to claim 1, wherein the transition metal is selected from the group consisting of iron (Fe), copper (Cu), manganese (Mn), and a combination thereof. 